1. Field of the Invention
The present invention relates to a thermoplastic extrusion assembly which provides a substantially homogeneous product, preferably a foam thermoplastic product, and which produces said product at a desired extrusion temperature and at an increased throughput rate over what is normally available, without requiring that a complex, costly and/or oversized extrusion assembly be implemented and further, without requiring substantial modification to existing manufacturing devices and procedures that may be integrated therein.
2. Description of the Related Art
The field of art associated with thermoplastic extrusion, and thermoplastic foam extrusion in particular, is quite specialized, and indeed, is quite different from that typically associated with metal, rubber, or non-foamed plastic extrusion. Specifically, foam extrusion generally requires an initial step of melting pellets, usually made of a thermoplastic material, and a subsequent step of mixing the melted thermoplastics with a foaming agent, such as a fluorocarbon (whether CFC, HCFC and/or HFC) or hydrocarbons (such as propane, butane, pentane, etc.), and possibly other agents such as nucleating agents, fire retardants and/or coloring agents, in an isolated extrusion environment. Moreover, the most effective foam extrusion techniques completely contain the extrusion material during the melting and mixing stages, maintaining the material in a non-foamed, viscous form until passed through an extrusion die and exposed to external forces. Indeed, it is when the extruded material exits the die of the foam extrusion assembly that it will foam (i.e. inflate and stiffen) into its ultimately usable form, such as films, planks, and large sheets from which meat trays, egg containers, small containers for butter and jelly, and the like, are formed. Accordingly, precision is imperative in order to ensure that an effective and complete mixing of the ingredients is achieved, thereby providing for a properly configured and homogeneous product, and further, to ensure that the entire extrusion system is well contained until the material passes through the die, thereby avoiding premature foaming of the extrusion material.
Of course, in addition to the above concerns associated with the formation of a foam product is the need to maintain the extrusion mixture at a rather precise extrusion temperature, corresponding to the polymer or substance being used as the basis for the extrusion mixture, so as to achieve a proper viscosity of the extrusion mixture and permit proper forming of the extrusion mixture through a die, such as a profile die, tube die, sheet die, annular die, flat die or several other common types of dies. The rather precise extrusion temperature at which a desired range of viscosity is achieved is unfortunately, less than the initial xe2x80x9cmelt temperaturexe2x80x9d, i.e. temperatures at which the pellets of thermoplastic extrusion material are melted, but cannot be too much less than that initial xe2x80x9cmelt temperaturexe2x80x9d for reasons about to be explained. As such, a substantial balance must be maintained. For example, if the melted extrusion mixture is permitted to cool too much, it will become too viscous and will fail to achieve the desired product density, becoming unusable, and in general, it will not move effectively through the extrusion assembly, let alone, out through the die. Conversely, if the temperature of the melted extrusion material is too high, its viscosity decreases significantly and the material is not dimensionally stable or shapeable as it flows through and importantly, from the die.
As can be appreciated, any one of the previously mentioned factors can have a significant effect on the productivity rate associated with the various phases of thermoplastic foam extrusion. Accordingly, typical thermoplastic foam extrusion processes are often designed to maximize the regulation and control over each factor. For example, a typical foam extrusion process is broken down into two separate phases, and indeed, often requires two separate extrusion devices linked with one another. The first phase of the procedure typically involves the effective melting of extrusion material pellets, such as a thermoplastic material particularly suited for the foam desired, and the subsequent mixing of those material pellets with the foaming agent and other various agents, as needed. Usually, this first phase of the procedure is performed within a very large and elongated mechanical extrusion device wherein a thermoplastic extrusion screw, and more specifically a melt screw, urges the material pellets through an elongate barrel causing the generation of frictional heat, and also from which melting heat is being applied. Indeed, the surface of the thermoplastic extrusion screw is rather slippery by design (i.e. often chrome plated and polished) and the material moves through the barrel as a result of a frictional engagement or shear when the extrusion material frictionally contacts the barrel surface. Moreover, it is the shear effect combined with the heated barrel which provides for effective melting of the extrusion material.
In addition to ensuring that proper homogeneous mixing is achieved, the first phase is also limited by the need to establish and maintain an isolated system. In particular, the isolated system prevents premature foaming of the thermoplastic material, and ensures that sufficient heating energy is being applied in order to effectively melt the material and permit complete mixing thereof. Many advances have been achieved in the industry so as to maximize the flow-through rates attainable by or within this first phase of the thermoplastic extrusion process. Unfortunately, however, actual output of the extruded product is still limited to levels well below those achievable in this initial phase as a result of the requirements and limitations of the second phase of the foam thermoplastic extrusion process.
In particular, unlike conventional thermoplastic extrusion which primarily requires a homogeneous mix, foam extrusion also includes a second phase which requires the evenly distributed and uniform cooling of substantially all of the melted and mixed extrusion material to a point where it is at a necessary extrusion temperature and consistency throughout. Usually, this second phase of the procedure is also performed within a very large and elongated mechanical extrusion device wherein a central thermoplastic extrusion screw, such as a foam cooling screw with a helix or paddle type configuration, urges the melted extrusion material through an elongated barrel to effect a cooling of the material. Given the need for a rather precise temperature, however, significant limitations relating to the turn rate of the central cooling screw and to the heat extraction rate achieved apply to this second phase of the process. Specifically, the turn rate of the central or cooling screw is limited because of the need to minimize the heat which results from shearing of the extrusion material with the wall surface of the barrel and internally by the material itself. As such, an increased flow-through rate of high quality cannot be achieved merely by speeding up the rotation of the screw. Furthermore, one cannot indefinitely merely counter the excess shear heat that is generated by providing for faster cooling merely by decreasing the temperature of the barrel, because if the mixture cools too much, an optimal viscous flow of the extrusion mixture is not maintained and productive passage through the die is dramatically impeded. Also, merely increasing the size of the extrusion barrel in the cooling phase is not an effective solution as such an assembly would become excessively large, cumbersome and financially impractical due to using such a large scale factor. Large scale cooling stage. processes also involve potentially undesirable operation cost implications due to the typically longer product change-over time factor and the related materials costs of such large machines. Moreover, if one merely increases the size of the extrusion barrel in the cooling phase and the passage through the die is made long enough so as to allow temperature tempering of the extrusion mixture, the resistance to flow generated by such passage would detrimentally promote heat generation in the extrusion mixture within the barrel. Further, this restricted long passage would negate the normal tendency of the thermoplastic extrusion mixture to swell after passage from the die is retarded.
Accordingly, it is seen that one must balance the needs of a productive flow through rate with the requirements of practicality and an effective and evenly distributed cooling.
Many in the industry have nevertheless failed to recognize the above-described limitations. For example, some in the industry have sought to increase the productivity of the second or cooling phase of the extrusion process by increasing the amount of heat which is extracted at the extrusion barrel""s surface. Unfortunately, these procedures have proven ineffective because when an amount of heat is extracted so as to effectively cool substantially all of the extrusion material throughout, the perimeter layers of the extrusion material, which are in more direct contact with the barrel surface, cool excessively and no longer provide a satisfactory extrudate. Thus, a primary difficulty associated with this cooling phase is the fact that the quantities of the extrusion mixture which are closest to the shaft of the central screw do not get effectively cooled, as a majority of the heat that is extracted comes first from the extruded material located about the perimeter area within the barrel of the second mechanical extrusion device. For example, as heat is extracted, the perimeter quantities of the extrusion material continue to get cooler and cooler while the interior quantities gradually cool to the desired, rather precise extrusion temperature. This yields extrusion material which is not of uniform consistency for proper extrusion. Indeed, in this second phase, as well as in the first phase, it is also noted that complete homogenization of the extrusion mixture is sometimes lacking, and as a result the finished product can be of diminished quality.
Accordingly, it is seen that the overall productivity of the present industry is still limited by the cooling phase of the extrusion process. To date, the only effective means of ensuring an effectively cooled extrudable material is to provide a slow, lengthy, and gradual cool down process so as to thicken the extrusion material without overly cooling only portions thereof, and so as to achieve a longer mixing time for increased homogenization. The assembly of the present invention, however, addresses the problems and needs which remain in the art and is able to significantly increase the flow-through rate of extrusion material without compromising the quality of the finished product.
The present invention relates to a thermoplastic foam extrusion assembly of the type structured to produce an extruded foam product in any of a variety of extruded shapes. In particular, the foam extrusion assembly includes a melt region, which receives and melts a plurality of pellets, preferably thermoplastic material pellets, and a heat extraction region.
Preferably included at the melt region of the foam extrusion assembly is an agent addition assembly. Specifically, the agent addition assembly is structured to add a foaming agent to the melted or melting material pellets for complete blending/mixing and homogenization therewith. To this end, the foam extrusion assembly also includes a mixing assembly. The mixing assembly, which is also preferably included with the melt region, is structured to thoroughly mix the melted material pellets and the foaming agent, thereby achieving a purer, more homogeneous extrusion mixture.
Unlike the melt region, the heat extraction region of the present invention is structured to extract excess heat from the mixture of melted material pellets and foaming agents so that the mixture can achieve an extrudable temperature and consistency. Preferably, the heat extraction region includes an elongate barrel through which the mixture flows. The barrel, which is preferably substantially enclosed, includes at least one inlet and at least one outlet defined therein. The inlet is structured to receive the mixture of melted pellets and foaming agent therethrough, preferably from the melt region, for passage into the barrel.
Cooperatively disposed with the barrel of the heat extraction region is a heat extraction structure. Specifically, the heat extraction structure draws heat from the barrel, such as by liquid cooling, therefore removing heat from the mixture of melted pellets and foaming agent in order to ensure that the extrusion mixture arrives at the extrudable temperature range.
Disposed within the barrel of the heat extraction region is a thermoplastic extrusion screw. This extrusion screw is preferably a cooling screw axially disposed within the barrel and is structured to rotate therein when the barrel contains a quantity of the mixture of melted pellets and blowing agent. In order to urge the mixture of melted pellets and blowing agent towards the outlet of the barrel, the extrusion screw further includes at least one screw flight. The screw flight is preferably structured to wrap around the extrusion screw and thereby rotate upon rotation of the screw in order to urge the mixture towards the outlet of the barrel. In order to achieve effective, complete and evenly distributed cooling of the mixture of melted pellets and foaming agent, however, the screw flight of the present invention further includes at least one, but preferably a plurality of circulation channels defined therein. In particular, the circulation channels are generally precisely positioned in the screw flight so as to receive quantities of the mixture of melted pellets and foaming agent therethrough upon rotation of the extrusion screw. Accordingly, upon receipt of the quantities of the mixture of melted pellets and foaming agent through the circulation channel, an effective circulation of the mixture relative to the barrel is achieved and substantially all of the mixture of melted pellets and foaming agent will substantially uniformly come into close proximity with the barrel for effective and even heat extraction and cooling thereof.
Lastly, the foam extrusion assembly includes a die. In particular, the die is cooperatively disposed in fluid flow communication with the outlet of the barrel, and can take on any of a number of desired forms so as to receive the mixture of melted pellets and foaming agents, at the extrudable temperature, therethrough in order to form a desired shape. Indeed, it is upon exit from the die that the mixture begins to xe2x80x9cfoamxe2x80x9d and thereby produce the finished, extruded product.
It is an object of the present invention to provide a foam extrusion assembly which, during the cooling phase, substantially distributes the cooling effects throughout the extrusion material mixture disposed within the barrel, thereby more effectively, efficiently and uniformly rendering substantially all of the extrusion material at an appropriate extrudable temperature corresponding the polymer or substance being used as the basis for the extrusion mixture.
A further object of the present assembly is to provide an extrusion assembly which can be utilized with existing extrusion structures without substantial modification, yet which substantially increases the overall throughput rate to be achieved to levels as high as generally about a ten to fifty percent increase over currently available through put rates, without sacrificing product quality.
Another object of the present invention is to provide an extrusion screw which can be effectively utilized in existing extrusion assemblies, and yet which more effectively provides for uniform heat extraction from hot and melted extrusion material, without overly cooling the material or heating the material, while at the same time, still substantially increasing the flow through rate to be achieved.
An added object of the present invention is to provide a foam extrusion assembly which substantially overcomes the limitations commonly associated with the cooling phase of the extrusion process.
Also an object of the present invention is to provide a foam extrusion assembly which effectively increases the extrusion production rate without significantly increasing the size and/or complexity of the structure to impractical and uneconomical levels.
Yet another object of the present invention is to provide a thermoplastic extrusion screw which effectively circulates the extrusion mixture and ensures that substantially, if not all of the extrusion mixture effectively and uniformly comes into proximity with the heat extracting surface of the barrel of the extrusion assembly.
Another object of the present invention is to provide a thermoplastic extrusion screw which substantially modifies the flow pattern normally exhibited by an extrusion mixture within an extrusion assembly, thereby providing more efficient and uniform processing thereof and achieving significant advances in the rate at which the extruded product is produced.
Still another object of the present invention is to provide a thermoplastic extrusion screw which effectively circulates the extrusion mixture and provides a substantially homogeneous and high quality product.